January 8, 2014

Invisible Light Patterns Help Bees Find Food Even On Cloudy Days

Bees are excellent navigators. Once they stumble upon a food source, they keep coming back to the same spot without faltering. They also have a great sense of smell and can recognize color patterns and symmetry in flowers – admirable feats for an insect whose brain is the size of a sesame seed.

Scientists have long known that bees use the sunlight like a compass to map their route to the flowers full of succulent dew. They also know that bees use a seemingly complex and deceptively random waggle dance to let their mates know where to find the food source. Scientists are now able to “understand” most of the dance movements, and have striven to figure out how bees translate the map in their tiny heads into these movements.

For some time now, scientists have also believed that bees use a pattern of light in the sky called polarized light in their navigation system. Polarized light is created from sunlight scattered in the sky by particles in the air, and is invisible to the human eye.

New research shows that bees use this polarized light to guide their movements, even when there’s no sunlight.

In the study, the researchers directed bees down a tunnel towards a sugar source. The tunnel blocked out sunlight and only polarized light was made to shine down on the bees from above. At times, the light was shining along the direction of the tunnel, at times at right angles to it. The researchers then watched to see how the bees waggled about the location of the food source to their mates.

In one experiment, where the polarized light was at right angles to the tunnel, the bees’ dance had a lot of upward and downward waggles, the researchers found. This meant that they were directing their mates to fly in direction of the polarized light. If they had been outside in sunlight, this would mean that they would have to fly in a direction towards or away from the sun.

When the light beam was lined up parallel to the tunnel, the bees’ dance had a lot of right and left waggles, pointing their mates to a straight line along the direction of the light.

“Other laboratories have shown from studying their eyes that bees can see a pattern of polarized light in the sky even when the sun isn't shining: the big question was could they translate the navigational information it provides into their waggle dance,” Mandyam Srinivasan, University of Queensland neuroscientist and senior author of the paper, said in a statement.

“The more we find out how honeybees make their way around the landscape, the more awed we feel at the elegant way they solve very complicated problems of navigation that would floor most people – and then communicate them to other bees.”

In another experiment, when one half of the tunnel had vertically shining light and the other half horizontal, bees used a combination of all four movements, showing that there were four equally likely food locations based on the two types of polarized light they picked up on in their route.

“When we shone polarized light that was across in the first part of the tunnel and in-line in the second part, we simulated for the bees an L-shaped journey in which they flew initially in a direction perpendicular to the sun (either to the left or to the right of it) and later along the direction of the sun (either towards or away from it). This created the dilemma that the food source could be in one of four possible locations. The bees addressed this dilemma by informing their nest mates to search in all four possible directions,” Srinivasan stated.

The researchers also zeroed in on the bees’ brain cells that get activated in the presence of polarized light. Two different pairs of brain cells get “switched on” depending on which direction the light is shining in – representing north-south or east-west, they found.

The study shows how bees are able to navigate and convey complex routes to their mates even when there’s no sunlight, despite having a brain that most biologists consider simple, the researchers conclude.